Organic material photodiode

ABSTRACT

The present invention relates to a photodiode including a photo-active layer, which layer includes at least one electron donating material, and at least one fullerene derivative as an electron accepting material. The present invention further relates to a method for making such a photodiode, to a photo-active layer and to a fullerene derivative.

This application is a continuation of PCT application no.PCT/NL04/00116, designating the United States and filed Feb. 17, 2004;which claims the benefit of the filing date of European application no.EP 03075461.8, filed Feb. 17, 2003; both of which are herebyincorporated herein by reference.

The invention relates to a photodiode, and in particular a solar cell,based upon an electron donating material and a fullerene as an electronaccepting material.

Solar cells typically comprise a number of photovoltaic cells.Traditionally, these cells are inorganic pn hetero-junction diodescomprising a layer of a p-type (electron donating) material and a layerof an n-type (electron accepting) material. Examples of materials forsuch inorganic diodes are silicon, gallium arsenide and cadmiumsulphide.

In WO 94/05045 it is proposed to make a photodiode from a conjugatedpolymer to form the layer of the p-type material and to use fullerene,in particular buckminsterfullerene (C60 fullerene), to form a layer ofthe n-type material. A disadvantage of a photodiode wherein the p-typematerial and the fullerene are present in separate layers resides in therequired thickness of the layers to provide an efficient photo-voltaiccell with sufficient light absorption. Due to the required thickness,the charge separation is found to be less efficient. Furthermore, theinternal electric resistance may be increased to an unacceptable level.

U.S. Pat. No. 5,759,725 relates to a photoconductor which comprises alayer of amorphous fullerenes, acting as a charge-generating material,not as an electron accepting material. The diode may comprise anon-conductive transparent matrix polymer. A solar cell is shown whereina film consisting of amorphous fullerene is present sandwiched betweenand in direct contact with the electrodes.

WO 01/84644 relates to a photovoltaic cell comprising two metalelectrodes disposed on both sides of a photo-active layer. Thephoto-active layer is built up out of a conjugated polymer as anelectron donating material and fullerene as an electron acceptingmaterial. The fullerene may be functionalised. The fullerene referred toin WO 01/84644 is Buckminster-fullerene (C60).

A problem often encountered with known fullerene based photo diodes,such as solar cells, is a relatively low efficiency and/or the fragilityof the fullerene layer In order to make the fullerene layer lessfragile, it has been proposed to prepare a heterojunction structurewherein the fullerenes react with another to form a polymeric fullerene,such as a fullerene dimer in EP 1 063 197.

There remains a need for alternative photo diodes, such as solar cells,that have a satisfactory efficiency and are preferably robust.Accordingly, it is an object of the present invention to provide a novelphoto diode, in particular a novel solar cell, that can serve as analternative to known photo diodes, respectively solar cells, with asatisfactory efficiency and robustness.

It has now been found that this object is realised by choosing afullerene from a specific group of fullerenes as an electron-acceptingmaterial.

Accordingly, the present invention relates to a photodiode, preferably asolar cell, comprising a photo-active layer which comprises at least oneelectron donating material (the p-type material) and at least onefullerene derivative (as an electron acceptor; the n-type material),which fullerene derivative comprises a carbon cluster of at least 70atoms and at least one addend attached to the carbon cluster, whichaddend is selected such that the fullerene derivative is compatible withor bound to the electron donating material.

FIG. 1 shows a schematic representation of a photodiode, such as solarcell.

It has been found that a photodiode according to the invention shows avery good efficiency, in particular in comparison to a similar cell withfullerenes having a carbon cluster of less than 70 atoms, such as C₆₀fullerene.

It has been found that a photo-active layer in a photodiode according tothe invention has an improved light absorbance in the range of 350-1000nm, which has been found to result in an increased photo-voltaic effect,in comparison to a comparable photo-active layer wherein the fullereneis a C₆₀ fullerene derivative (e.g. about sixfold for a C₇₀ incomparison to C₆₀).

Further, a photo-active layer in a photodiode according to the inventionhas been found to have very good electron-accepting properties.

In addition, it has been found that in combination with a good lightabsorbance, the electric conductivity is highly satisfactory.

Accordingly, a photodiode according to the invention has been found tobe very suitable for use in a variety of applications as an alternativeto known photo diodes. A photodiode has been found particularly suitablefor use as a solar cell, as a light intensity meter or as a photodetector, e.g. in an optical scanner, such as a flat bad scanner.Accordingly the present invention also relates to a solar cell, a solarpanel, a light intensity meter, a photo detector (such as an opticalscanner), comprising a photo diode as described in the presentdescription or claims. Suitable ways to employ the photodiode(s) in suchan apparatus are analogous to those for known photodiodes such as thosebased upon semi conductive polymers and C60 fullerene (derivatives).E.g. a photo detector may be made such as described in “Large-Area,Full-Color Image Sensors Made with Semiconducting Polymers”, G. Yu; J.Wang; J. McElvain; A. J. Heeger, Advanced materials 10 (1998), 17,1431-1433.

The p-type material, e.g. a conjugated oligomer or polymer, and then-type material may be present in the photo-active layer as a mixture ormay be present as separate sub-layers of n-type material respectivelyp-type material. Very good results with respect to stability androbustness of the system and the efficiency have been achieved with amixture of p-type and n-type material.

Fullerenes are clusters of covalently bonded atoms, mostly or all carbonatoms, providing a three-dimensional structure (also referred to as thecarbon cluster). Typical structures are more or less spherical shapes(such as the “soccer-ball” shaped C₆₀ fullerene and the more oval C₇₀fullerene) and cylindrical shapes (such as the tubular C₅₀₀ and C₅₄₀fullerenes). The fullerene moiety of a fullerene derivative in aphotodiode, such as a solar cell, according to the invention may be anyfullerene comprising such a carbon cluster formed by at least 70 atoms.For practical reasons the amount of atoms forming the carbon cluster ispreferably 960 or less, more preferably 240 or less, even morepreferably 96 or less, e.g. C₇₀, C₇₆, C₇₈, C₈₀, C₈₂, C₈₄, C₈₆, C₈₈, C₉₀,C₉₂, C₉₄ or C₉₆.

Optionally a photodiode, such as a solar cell, comprises severaldifferent fullerenes, e.g. a mixture of fullerenes with a differentamount of atoms and/or several isomers of a fullerene. It is alsopossible that a fraction of the fullerenes in the photodiode isunderivatised. For example, from a practical view point it may bepreferred to manufacture a cell according to the invention from a batchof derivatised fullerenes, wherein a residual amount of unreactedfullerenes is still present.

Preferably the photoactive layer comprises a fullerene derivativewherein the carbon cluster is formed by at least 76 atoms. It has beenfound that with such a fullerene a particularly good light absorptioncan be realised.

Preferably, essentially all atoms forming the carbon cluster are carbonatoms, although it is possible that a minority of the atoms is adifferent atom, e.g. a nitrogen, oxygen, sulphur or boron. Very goodresults have been obtained with a fullerene wherein 0-6 of the atomsforming the carbon clusters are atoms other than carbon. Optionally, thecarbon cluster comprises one or more atoms inside the interior space ofthe carbon cluster, such as one or more lanthanides and/or alkali atoms.

The carbon cluster may form a single three-dimensional structure withone central space (referred to as a single cage, a single fullerene unitor a fullerene monomer), a fullerene-oligomer essentially consisting ofseveral cages (e.g. a fullerene-dimer) or a fullerene-polymeressentially consisting of fullerene cages. Very good results have beenachieved with a photodiode, in particular a solar cell, wherein thefullerene is a fullerene monomer. The term oligomer is used herein todescribe a moiety comprising two to nine monomeric units (e.g. fullerenecages or organic monomers). The term polymer is used herein to describea moiety comprising more than nine monomeric units.

The fullerene derivative further comprises at least one addend attachedto the carbon cluster. The term addend is used herein to describemoieties that are compatible with the electron donating material, suchthat the fullerene derivatives adheres to the electron donating materialwhen applied there to or such that the fullerene derivative is misciblewith the electron donating material to form a mixed phase.

Thus, the term compatible is used to indicate that in combination withthe p-type material the n-type material is miscible to form aphoto-active layer with a suitable functionality to act as aphoto-active layer. Typically, the compatibility contributes to theformation of an interpenetrating network of any kind. In particular, thefullerene derivative is considered to be compatible with the electrondonating material when a mixture of the fullerene derivative and theelectron donating material, present in a photoactive layer in aphotodiode is capable of absorbing photons, then generating chargecarriers (such as electrons and/or holes) and allowing migration of thecharge carriers to an electrode, connected to the photoactive layer inan electrically conductive manner.

The mixed phase may be homogenous on a macro-scale. On a nano-scale itmay be phase-separated and contain different phases which phases maydiffer in the ratio of p-type to n-type material. Macroscopically, themixture may have a more or less uniform ratio throughout the thicknessof the photo-active layer or the layer may be stratified, i.e. a layerwherein the ratio of p-type to n-type material changes gradually orstepwise throughout the thickness of the photo-active layer.

Preferably, the volume of the addend(s) is less than that of thefullerene(s). Thus it has been found possible to maintain the electrontransport properties particularly well.

Examples of suitable addends are linear, branched or cyclichydrocarbons. A hydrocarbon is optionally substituted and optionallycontains one or more contains functional groups, e.g. selected from thegroup consisting of carboxylic esters, amides, ethers, lactones,lactams, urethanes, carbonates, acetals, amines and halogens.

A preferred addend comprises an alkyl group, more preferably an alkylgroup with 1-12 carbon atoms, even more preferably with 2-7 carbonatoms. The alkyl groups may be linear, branched, or cyclic. Preferablyat least the majority of the alkyl groups are linear. The alkyl groupsmay bear one or more substituents and/or functional groups. Asfunctional groups, ethers and carboxylic esters with alkyl moieties ofpreferably 1-10 carbon atoms are particularly preferred. An addend mayalso be or contain one or more aryl groups. Examples of suitable arylgroups are those based on benzene, thiophene, pyrrole, pyrrolidine andfurane. The aryl groups themselves may be substituted with one or moreside chains. Preferred side chain are those selected from the groupconsisting of alkyl, alkoxy, alkoxycarbonyl, C- and/or N-alkylamido andaryl groups.

Very good results have been achieved with a (methano)fullerenederivatised with an addend comprising an aryl group plus an esterifiedalkane carboxylic acid group, and functional analogues of such a(methano)fullerene. The aryl may for example be phenyl, thiophene,indole, pyrrole or furan, of which phenyl is particularly preferred. Theester may be formed of a linear or branched C1-C20 carboxylic acid,preferably a C3-C7 carboxylic acid, more preferably a C4 carboxylic acidand a C1-C20 alkanol, preferably a C1-C4 alkanol. A highly preferredderivatised (methano)fullerene of this type is phenyl-butyricmethylester derivatised fullerene (also referred to as [n]-PCBM whereinn is the number of atoms forming the carbon cluster).

For improving the voltaic properties of a photodiode, comprising afullerene derivative comprising an aryl-(carboxylic acid alkyl)estermoiety, it has been found advantageous to substitute at least one of thehydrogens of the aryl (in particular the phenyl) with a carboxylic acidgroup, preferably a methoxy group. For improving the compatibility of afullerene derivative comprising an aryl-(alkoxy alkyl)ester moiety, ithas been found advantageous to substitute one or more of the hydrogensof the aryl with an alkyl or alkyl-like moiety.

Preferred derivatives of the PCBM type include (methano)fullerenesderivatised at the [6,6] position of (methano)fullerene and derivatisedthat are derivatised at the [5,6] position, which are known as fulleroidor homo-fullerene.

Another group of particularly suitable fullerene derivatives are thoseselected from the group consisting of N- and/or C-substitutedfullereropyrrolidines”, “Diels-Alder adducts”, “N-substituted[5,6]azafulleroids”, “N-substituted ketolactams, such as those obtainedby photooxygenation of [5,6]azafulleroids”, “N-substitutedfulleroaziridines”;

It is not necessary that the addend absorbs photons in the VIS range(which is the case with dyes). In fact, it has been found that manyvisible light absorbing dyes tend to be detrimental to the electricconductivity of the photo-active layer. In particular in case a veryhigh electric conductivity is desired, it is therefore preferred thatthe addend is essentially transparent to visible light. In practice anaddend is considered transparent in case the extinction coefficient inthe range of 400-800 nm of about 500 cm⁻¹mol⁻¹ or less.

The carbon cluster may comprise an addend at one or more of the atomsforming the cluster. Preferably the number average of fullerene-addendbonds per fullerene is in the range of 1-3.

In principle, the electron donating material can be any inorganic ororganic material having electron donating properties when present in thevicinity of a fullerene derivate. For example, an electron donatingmaterial may be chosen from the group consisting of op p-type(semi)-conducting molecular materials, preferably p-type conjugatedpolymers, p-type conjugated oligomers, p-type conjugated moleculeswithout repeating units (i.e. non-polymeric, non-oligomeric molecules),quantum dots & wells and inorganic semi-conductive nano-particles.

Particular suitable examples of electron donating molecules withoutrepeating units include prophyrins, phtalocyanines, (both either with orwithout a metal atom or ion complexed), and substituted coronenes.

Particular suitable examples of quantum dots and wells include thosedescribed in Luque, A.; and Marti, A.: Increasing the Efficiency ofIdeal Solar Cells by Photon Induced Transitions at Intermediate Levels.Phys. Rev. Lett., vol. 78, no. 26, 1997, pp. 5014-5017 and in Murray, C.B.; Norris, D. J.; and Bawendi, M. G.: Synthesis and Characterization ofNearly Monodisperse CDE (E=S, SE, TE) Semi-conductor Nanocrystallites.J. Am. Chem. Soc., vol. 115, no. 19, 1993, pp. 8706-8715.

Particular suitable examples of nanoparticles include nanocrystallineCuInS₂ and the like, e.g. as described in “Photovoltaic properties ofnanocrystalline CuInS2-methanofullerene solar cells”, Elif Arici; N.Serdar Sariciftci; Dieter Meissner, Molecular crystals and liquidcrystals science and technology, vol. 385 (2002), pag. 129 or nanoparticles as described in Hybrid Nanorod-Polymer Solar Cells Wendy U.Huynh, Janke J. Dittmer, A. Paul Alivisatos, Science 295, 2425-9 (2002).

Preferred examples or p-type oligomers and polymers are oligomersrespectively polymers of derivatised and underivatised thiophenes,phenylenes, fluorenes, acetylenes, isothionaphtenes, benzthiaziazoles,pyrroles and combinations thereof. A particular preferred combination isa p-type material selected from the group consisting of (phenylenevinylene) oligomers and polymers.

Suitable combinations include blends, copolymers and hybrid structurescomprising said p-type material. In particular with respect toprocessibility very good results have been achieved with a compoundselected from the group consisting of oligo- and polyalkylthiophenes,oligo- and poly(dialkoxyphenylene vinylene)s, oligo- andpoly(9,9-dialkylfluorenes) and with oligo- and poly(N-alkylpyrroles).

A highly preferred group of derivatives are the oligo- andpoly(dialkoxyphenylene vinylene)s and in particular oligo- andpoly[[2-methoxy-5-(3′,7′,-dimethyloctyloxy)]-p-phenylene vinylene](MDMO-PPV).

A suitable ratio of fullerene derivative to p-type material, such asp-type polymer or oligomer, can routinely be determined for a particularcombination of materials, based upon information disclosed herein andcommon general knowledge. Good results have for example been achievedwith a photodiode, in particular a solar cell, wherein the p-typematerial to fullerene ratio (weight to weight) is about 10:1 to 1:10,preferably 1:1 to 1:5. Very good results have been achieved with a ratioof about 1:2 to 1:4. In practice, the photo-active layer preferablyessentially consists of n-type material, in particular fullerenederivative, and p-type material.

FIG. 1 shows a schematic representation of a photodiode according to theinvention. The outermost transparent layer 1 (facing the sun light whenoperational) serves as the substrate for the other layers. It may bemade of any material through which sun-light (visible, UV and/or nearIR, preferably at least visible and near IR) can pass and that issufficiently stable when exposed to such light. Suitable materials forthe outermost layer are known in the art. Preferred examples of suchmaterials include glass, and transparent polymers, in particularplastics, such as poly ethylene therephtalate (PET). The photoactivelayer 4 is positioned between two electrodes 2 and 6, one of whichserves as the anode and the other as cathode. For practical reasons, thecathode is preferably positioned between the layer 1 and the photoactivelayer 4. In case the anode is sufficiently transparent, the anode may bepositioned between layers 1 and 4. Suitable materials for the cathodeand the anode are known in the art and the skilled person will know howto choose a suitable combination. A preferred cathode is selected fromthe group consisting of Transparent Conducting Oxides, of which SnO₂:F,SnO₂:Sb, ZnO:Al, and Indium-Tin-Oxide (ITO) are particularly favouredexamples. These show favourable transparency to allow passage of thephotons to the photo-active layer. Suitable anodes include calcium,aluminium, barium, gold, platinum or silver. In principle it is possibleto use a thin metal layer as the electrode between layer 1 and thephotoactive layer 4, provided that the layer is (semi-)transparent.Optionally, the photodiode comprises one or more other layers. Suitableother layers have been reported ubiquitously in the art. Examplesthereof are a poly(3,4 ethylene dioxythiophene)polystyrene-sulphonate(PEDOT:PSS) layer 3, or a functional analogue of that material and/or aLiF layer 5 or a functional analogue thereof.

The dimensions of the various layers may be chosen within wide ranges.The skilled person will know how choose suitable values, depending uponthe chosen materials and the desired specifications for the cell. Goodresults have for example been achieved with an electrode 2 thickness inthe range of about 20 to 500 nm (in particular with ITO), a PEDOT:PSSlayer 3 in the range of about 20 to 500 nm, a photoactive layer 4 in therange of about 20 to 1000 nm, a LiF layer 5 in the range of about 0.1 to10 nm and a electrode 6 in the range of about 20 to 1000 nm.

The present invention further relates to a method for preparing aphotodiode, in particular a solar cell, wherein at least one p-typematerial, preferably a (semi)-conductive polymer, and at least onefullerene as defined herein are mixed with a liquid and thereafter driedto form the photo-active layer.

The present invention further relates to a method for preparing aphotodiode, in particular a solar cell, wherein a photo-active layer ismade by applying a sub-layer, comprising at least one electron donatingmaterial as defined herein to a substrate (usually including one of theelectrodes and optionally a layer such as PEDOT:PSS) and applying aseparate sub-layer comprising at least one fullerene derivative, asdefined herein, to the substrate.

The p-type material, the n-type material, respectively a mixturethereof, can be applied by any means known in the art, suitable for thespecific p-type material, e.g. by spin-coating, ink-jet printing, doctorblading, spray coating, solvent casting in case of a p-type polymer orp-type oligomer.

It has surprisingly been found that the fullerene-derivative cansuitably be applied directly from a solution of thefullerene-derivative, e.g. by spin-coating, dipping, pouring andegalising. In contrast thereto, fullerene based n-type layer in aphotodiode, such as a solar cell, described in the prior art, istypically made by a vaporising technique, which tends to be difficultand expensive.

The present invention further relates to a fullerene derivative asdescribed herein, in particular to a fullerene derivative wherein thecarbon cluster is formed of 70-960 atoms, more preferably of 76-240atoms, even more preferably of 86-240 atoms. More in particular theinvention further relates to a fullerene derivative of the type[n]-PCBM, as defined above, wherein n is the number of atoms forming thecluster. Other preferences are as indicated herein for thefullerene-derivative in a photodiode, as described herein. A fullerenederivative according to the invention, may be made in a manneranalogously to the preparation of a corresponding C60 fullerene.

The present invention further relates to the use of a fullerenederivative as described herein, for improving the photo-voltaic effect,in particular for increasing photo-voltaic power conversion efficiencyin a photo-active layer, preferably for improving the light absorbingand/or electron accepting properties.

The invention will now further be illustrated by the following examples.

EXAMPLE 1

The preparation was carried out analogously to the procedure describedin S. E. Shaheen et al. Appl. Phys. Lett. 78, 841-843 (2001), wherein aC60 fullerene was used.

Poly [2-methoxy, 5-(3′,7′-dimethyl-octyloxy)-p-phenylene-vinylene](MDMO-PPV=) was used as the electron donor and [6,6]-Phenyl-butyric acidmethyl ester derivatised C70 fullerene ([70]-PCBM) as the electronacceptor. ITO/glass substrates were used as front electrode. A PEDOT-PSSlayer (Bayer AG, EL-Grade) of about 100 nm was spin cast on top of theITO layer from an aqueous suspension. The photo-active layer (about 80nm) was spin cast from a 1,2-dichlorobenzene solution with a 1:4 weightratio of MDMO-PPV and PCBM. As top-electrode, a thin LiF about 1 nm andsubsequently a 100 nm Al layer were deposited on top of the organiclayers by thermal evaporation.

For comparative reasons another cell was made in the same manner butwith [60]-PCBM as the electron accepting material.

The efficiencies of both cells were determined as described in “Accurateefficiency determination and stability studies of conjugatedpolymer/fullerene solar cells”, J. M. Kroon, M. M. Wienk, W. H. Verhees,J. C. Hummelen, Thin Solid Films 403-404, 223-228 (2002)). Theefficiency of the solar cell with [70]-PCBM was 20% higher than theefficiency of the collar cell with [60]-PCBM (3.0% instead of 2.5%).

1. A photodiode, comprising a photo-active layer which layer comprises amixture of at least one electron donating material and an isomericmixture of a fullerene derivative comprising a carbon cluster of atleast 70 atoms and at least one addend bound to the carbon cluster,which addend is selected such that the fullerene derivative iscompatible with the electron donating material, wherein the degree ofderivatisation of the fullerenes is 1 to 3 fullerene-addend bonds perfullerene.
 2. A photodiode, comprising a photo-active layer which layercomprises a mixture of at least one electron donating material and anisomeric mixture of a fullerene derivative comprising a carbon clusterof at least 70 atoms and at least one addend bound to the carboncluster, which addend is selected such that the fullerene derivative iscompatible with the electron donating material, wherein the electrondonating material is selected from the group consisting of conjugatedpolymers, conjugated oligomers, conjugated molecules that are free ofrepeating units, quantum dots, quantum wells and inorganicsemi-conductive nanoparticles.
 3. The photodiode according to claim 2,wherein the conjugated polymer is selected from the group consisting ofpolymers of thiophenes, phenylenes, fluorenes, acetylenes,isothionaphtenes, benzthiaziazoles, pyrroles and combinations thereof.4. The photodiode according to claim 2, wherein the conjugated oligomeris selected from the group consisting of oligomers of thiophenes,phenylenes, fluorenes, acetylenes, isothionaphtenes, benzthiaziazoles,pyrroles and combinations thereof.
 5. A photodiode, comprising aphoto-active layer which layer comprises a mixture of at least oneelectron donating material and an isomeric mixture of a fullerenederivative comprising a carbon cluster of at least 70 atoms and at leastone addend bound to the carbon cluster, which addend is selected suchthat the fullerene derivative is compatible with the electron donatingmaterial, wherein the weight to weight ratio of fullerene to p-typematerial in the photo-active layer is in the range of 10:1 to 1:10.
 6. Asolar panel, comprising a photodiode comprising a photo-active layerwhich layer comprises a mixture of at least one electron donatingmaterial and an isomeric mixture of a fullerene derivative comprising acarbon cluster of at least 70 atoms and at least one addend bound to thecarbon cluster, which addend is selected such that the fullerenederivative is compatible with the electron donating material, whereinthe electron donating material is selected from the group consisting ofconjugated polymers, conjugated oligomers, conjugated molecules that arefree of repeating units, quantum dots, quantum wells and inorganicsemi-conductive nanoparticles.
 7. A photo detector or light intensitymeter, comprising a photodiode, comprising a photo-active layer whichlayer comprises a mixture of at least one electron donating material andan isomeric mixture of a fullerene derivative comprising a carboncluster of at least 70 atoms and at least one addend bound to the carboncluster, which addend is selected such that the fullerene derivative iscompatible with the electron donating material.
 8. A photodiode,comprising a photo-active layer which layer comprises a mixture of atleast one electron donating material and an isomeric mixture of afullerene derivative comprising a carbon cluster of at least 70 atomsand at least one addend bound to the carbon cluster, which addend isselected such that the fullerene derivative is compatible with theelectron donating material, wherein the fullerene derivative is amethanofullerene.